While efforts have actively been made toward environmental issues and energy issues, expectations of thermoelectric conversion technology have grown under such circumstances. Because heat is the most common energy source that is available from various situations, such as body heat, solar heat, engines, and industrial exhaust heat, thermoelectric conversion is expected to become more important in future for efficiency enhancement in energy use for a low-carbon economy or for applications of power supply to ubiquitous terminals, sensors, or the like.
A wide variety of heat sources, such as body heat of humans or animals, lighting (fluorescent lamps and street lamps), IT equipment (display server), automobiles (peripheral area of engine and exhaust pipe), public facilities (waste incinerators and water service pipes), buildings (walls, windows, and floors), and natural structures (plants, rivers, and grounds), can be used for thermoelectric conversion apparatuses. In thermoelectric conversion, a device should be brought into intimate contact with such a heat source, and a generated temperature difference should be used efficiently. However, most of heat sources have curved surfaces or irregularities. Therefore, it is desirable for a thermoelectric conversion apparatus to have flexibility so that it can readily be provided on heat sources having various shapes.
However, a thermoelectric conversion apparatus generally has a complicated structure in which a large number of thermocouples having a p-n junction are arranged and connected to each other. Therefore, it has been difficult for a thermoelectric conversion apparatus to adopt a flexible structure in view of the reliability and the like.
As a known method of providing a flexible thermoelectric conversion apparatus, for example, JP-A 2004-104041 (Patent Literature 1) discloses a method of arranging a plurality of thermocouples with use of a flexible polymer sheet as a support member. Furthermore, JP-A 2003-282970 (Patent Literature 2) discloses a structure in which a plurality of thermoelectric conversion devices of a p-type element and an n-type element stacked via a thermal insulator and cross-linked to each other with an electrode are jointed on a substrate having a flexible polymer sheet. Moreover, JP-A 2010-199276 (Patent Literature 3) describes a method of forming a flexible thermoelectric conversion apparatus by patterning p-n junctions with a coating or printing method.
The aforementioned methods can implement a flexible thermoelectric conversion apparatus. Nevertheless, if even one junction or wire is broken in a structure having a large number of thermocouples being connected, the function of thermoelectric power generation is impaired. Accordingly, there are still problems in a highly reliable operation of a flexible device.
Furthermore, in recent years, there has been discovered the spin Seebeck effect, which generates currents of spin angular momentum (spin currents) when a temperature gradient is applied to a magnetic material. JP-A 2009-130070 (Patent Literature 4), JP-A 2009-295824 (Patent Literature 5), Nature Materials 9, 894 (2010) (Non-Patent Literature 1), and Appl. Phys. Lett. 97, 172505 (2010) (Non-Patent Literature 2) show a structure in which spin currents induced in a magnetic body by the spin Seebeck effect are derived as an electric current (electromotive force) by a metal electrode.
Major features of a thermoelectric conversion apparatus using such a spin Seebeck effect include the fact that a thermoelectric conversion apparatus can fundamentally be formed merely by a junction structure of a magnetic body and an electrode and the fact that no complicated thermocouple structure is needed. Therefore, such a thermoelectric conversion apparatus may remarkably reduce a probability of occurrence of deficiencies such as the aforementioned breakage as compared to a conventional thermoelectric conversion apparatus having a large number of thermocouple junctions. Thus, such a thermoelectric conversion apparatus is expected as a flexible thermoelectric conversion apparatus having high reliability.
However, even if a thermoelectric conversion apparatus uses the spin Seebeck effect to simplify a device structure like a thermoelectric conversion apparatus illustrated in Patent Literatures 4 and 5 and Non-Patent Literatures 1 and 2, there is still room for improvement in producing a flexible device having high reliability. Specifically, the inventors' experiments have revealed the following. In a case of a structure in which a magnetic film and an electrode are deposited on a substrate, even if a flexible substrate is adopted, a high stress is applied to the magnetic film and the electrode when a device is bent. As a result, the thermoelectric conversion function may be impaired by breakage of the magnetic film or the electrode, separation of the junction portion of the magnetic film and the electrode, and the like.
Furthermore, the following has also been revealed. If the magnetic film or the electrode should be broken, such a high stress is applied directly to the magnetic film and the electrode, resulting in a large scattering loss of spin currents, for example, in a junction of the magnetic body and the electrode. Therefore, a thermoelectric power may be reduced.